The invention relates to a method of manufacturing a low-pressure mercury vapour discharge lamp comprising a discharge vessel which is closed in a vacuum-tight manner and which comprises two or more substantially parallel co-extending glass discharge tube portions, the discharge spaces of two adjacent discharge tube portions being interconnected by a cross-coupling extending transversely to the wall of those tube portions, while during operation of the lamp the discharge passing through the greater part of the tube portions and the coupling in which method the coupling is provided after the inner wall of the tube portions has been provided with a luminescent layer and the tube portions have been closed at least at one end. The invention further relates to a low-pressure mercury vapor discharge lamp manufactured by means of this method. Such a method is known from the Netherlands Patent Application No. 7,902,571 to which U.S. Pat. No. 4,324,447 corresponds.
In the method described in the above mentioned Patent Application, the said tube portions are first provided with a luminescent layer. Subsequently, a mount (supporting an electrode and an exhaust tube) is secured to a first end of a tube portion and the other ends are closed, for example, by sealing. A luminescent layer is likewise present on the inner side of the closed ends. Subsequently, the coupling is provided.
The closure of the end of a tube coated with a luminescent material involves the risk of particles of the luminescent material being enclosed in the glass. The possibility (after the glass has solidified) of the occurrence of leakage and even of fracture is a real one. Additionally, stresses are produced in the glass near the closed end during the (later) step of providing the coupling between adjacent tube portions, which coupling engages at a comparatively short distance from this closed end, as a result of which there is a great risk of fracture in the glass at the area of the seal.